A broad range of physical experiments and industrial applications have been proposed by employing optical microresonators. In this context, modelling, which is the main way of informing design decisions, is of paramount importance and still presents challenges when dealing with multiphysics stiff systems. Along this line, this paper presents a reliable numerical model for the scalable reduction of a whispering-gallery mode resonator, by which its dynamic response subject to optical forces can be easily simulated. Moreover, additional thermal and mechanical modes can be added up to the desired level of approximation and thermal noise can be enabled.